Alarm systems, such as burglar and fire alarm systems, typically include one or more centralized control panels that receive information from various sensors distributed throughout a structure, building, area, house, etc. For example, a typical household burglar alarm system may include a plurality of magnetic contacts, motion sensors, and vibration sensors that are grouped into independently-monitored detection circuits connected to a control panel. Each detection circuit may be installed in a designated area or “zone” within a building. During normal operation of the alarm system, the control panel may monitor a voltage signal in each detection circuit for variations that may represent the occurrence of a particular alarm condition within a zone. For example, an elevated voltage signal in a particular detection circuit may represent the detection of a broken window by a vibration sensor in a corresponding zone of the house, and may cause the control panel to enter an alarm mode. Since the physical location of each zone of an alarm system is known, homeowners and response personal are readily apprised of the area of the house in which the alarm condition originated.
Like many other electrical systems, alarm systems are sensitive to various types of electromagnetic interference or “noise.” Noise refers to the unintentional or unwanted introduction of energy of a specific frequency, or range of frequencies, into an electrical signal. A predominant cause of electrical signal noise in alarm systems is so-called “line frequency noise.” Line frequency noise (and noise stemming from its harmonic frequencies, which occur at integer multiples of the underlying line frequency) in alarm systems is a byproduct of the near ubiquitous presence of generated electricity and devices powered by electricity located near various components of an alarm system. In the United States, for example, electrical power is distributed in the form of an alternating current (AC) that cycles at 60 Hz, which can produce noise at 60 Hz and at harmonics thereof. Such noise is commonly referred to as “AC induction noise.”
Because electrical noise can introduce unwanted components into an electrical transmission signal, it can create significant problems in the context of alarm systems where various sensors communicate with a control panel. For example, line noise may interfere with voltage signals associated with detection circuits. Thus, unless the part of a signal that is attributable to noise is filtered out, a control panel of an alarm system may incorrectly interpret the operational state of a zone in the system. For example, the presence of noise in a detection circuit may cause a control panel to record an alarm condition in a particular zone when no such condition exits, creating a so-called “false alarm.” In another example, the presence of noise could cause a control panel to erroneously record a fault condition, such as may be indicative of a system malfunction, when an alarm condition actually exits. Such inaccurate reporting runs counter to the fundamental requirements of most alarm systems, which dictate that an alarm system should not false alarm and should operate as intended when subjected to an alternating current induced on any signal lead, sensing lead, loop, DC power lead, or any other lead that extends throughout the wiring of a monitored premises.
In certain alarm systems, hardware signal filters are used to remove unwanted electrical noise from monitored voltage signals. For example, conventional notch filters are commonly employed in alarm systems to attenuate signal frequencies at or near 60 Hz which are known to be associated with AC induction noise. These frequencies are thereby ignored by a control panel and are prevented from affecting proper operation of the alarm system. Although hardware signal filters can be effective for improving the robustness of alarm systems to the extent they filter out AC induction noise, they have several shortcomings and disadvantages. For example, the additional circuit components necessary for implementing hardware signal filters can significantly increase the cost of an alarm system. Since each detection circuit in an alarm system must generally be associated with a separate hardware signal filter, the cost of implementing filters in systems having a large number of zones can be substantial.
Another shortcoming associated with hardware signal filters is that they can be slow to respond to the introduction of signal noise. For example, a typical control panel will record an alarm condition in a zone if a voltage signal representative of such a condition persists in the detection circuit of the zone for longer than a predetermined “loop response time,” usually ranging from of about 30 milliseconds to about 2.5 seconds. In the case of so-called “fast loop response zones,” which typically have response times of about 30 milliseconds, an alarm condition induced by signal noise may be recorded by a control panel before a conventional hardware filter is able to remove the unwanted signal frequencies, thus causing a false alarm. Additional circuit components can be added to alarm system circuitry to improve the speed with which hardware filters are able to counteract noise, but such components further increase the complexity and cost of typical alarm systems. A further disadvantage associated with hardware signal filters in alarm systems is that such filters are generally difficult to install and, once installed, are difficult to modify. For example, disabling or altering the operational characteristics of a hardware filter generally requires the physical alteration of alarm system circuitry, necessitating trained personal as well as significant additional time and expense.
In view of the foregoing, it would be advantageous to provide an alarm system having noise filtering means, and particularly AC induction noise filtering means, that are inexpensive, fast-responding, and that can be easily implemented.